


The Butterfly Effect

by MarshmarrowSans



Series: Tumblr Requests [4]
Category: Undertale (Video Game)
Genre: Don't be fooled by the fancy title this is just pwp, Established Relationship, F/M, Femdom, Light Bondage, Multiple Orgasms, No Ecto-Penis (Undertale), Sans on the other hand nuts three times, Sensitive bones, Sub Sans, You don't get to nut, dom reader, reader is female
Language: English
Status: Completed
Published: 2017-11-02
Updated: 2017-11-02
Packaged: 2019-01-28 11:59:52
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,168
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/12606120
Author URL: https://archiveofourown.org/users/MarshmarrowSans/pseuds/MarshmarrowSans
Summary: You didn't think you would be able to take the sight and sound of a vibrator taped to your skeleton boyfriend's crotch seriously.You were wrong.





	The Butterfly Effect

**Author's Note:**

> We got a request for... basically what you see here. Sub Sans + light bondage + vibrators + multiple orgasms = one hell of a fanfic in my book. Hope you guys enjoy it as much as I enjoyed writing it!

The silk of what was usually your favorite autumn scarf felt soft on your skin as you stretched it out between your hands.  It was about to take on a whole different purpose, one very different from keeping the chill off your neck.  You didn't need it for that-- you had your boyfriend's jacket resting on your shoulders.  You just wanted to use that scarf instead of rope, for your first time doing something like this with your skeletal boyfriend.  You felt like it fit your personality-- strong, but soft.  Holding him in place, but giving him solace.  You secured him, but you weren't stifling or rough, not at all.  You were comfortable.  You were safe.

 

"A small change in initial conditions can result in drastic changes in the output," you mused in a soft purr as you contemplated your options.  You only had one scarf, and so, so many fun places you could tie it.  You have to be economical about this.  Slowly, you devised a way to tie his wrists together, and to the headboard, all with the same length of fabric.

 

"and what does that have to do with you, uh, tying me to our bed right now?" your boyfriend asked with a hint of humor in his voice.  It was nice, at least, to see that he wasn't apprehensive about this.

 

"Tying you up is the small change in initial conditions.  The crazy orgasms you're gonna have as a result are the dramatic change in the output."  You smiled back down at him eagerly.  "But honestly?  I mostly just brought it up because it's called the butterfly effect...  and this little butterfly," you proudly presented him with the little vibrator that had been your best friend before he came along, "is about to have one hell of an effect on you."

 

"heheheh.  you're learning from the pun master.  that's great."  Sans looked curiously at what you had in your hand.  "what's that?  you said it was a butterfly?  looks like a dick."

 

"It's a vibrator.  And I'm about to show you what it can do."  You gave the scarf a firm tug to test its integrity.  Good.  The knot you'd tied was secure, and the fabric didn't seem likely to tear unless you brought scissors to it.  "You said you wanted to try having an orgasm without using your magic?"

 

Sans flushed and glanced away from you.  "well, it was your idea, but i, uh, didn't object to it..." he answered tepidly.  You weren't about to accept that for an answer.  You wanted honest, open, enthusiastic consent.  You slipped his shorts over his hip bones and hooked your fingers through the gaps at his backside to slide him closer to you.  He didn't resist in the least, even as his arms stretched over his head, held in place by the bedposts.

 

"But you want it, right?" you pressed him.  "You want me to touch you and play with you until your body doesn't give a shit anymore if it has something to cum with, you're just going to be in total ecstasy regardless."  You could feel his magic rush through him in response to your interrogation. It heated the bones from his core all the way to his phalanges, but especially those in his pelvic region.

 

"you're damn right i do," he breathed.  Now that was what you were looking for.  
  
"Good.  Because that's the plan for tonight.  Now just relax while I get this set up."

 

"ooh.  i don't mean to brag, baby, but i'm really good at that."  Sans' eyes closed and his head lulled back with a cheeky grin.  A part of you couldn't wait to wipe it off his face, like you always did during sex.

 

This part, you hadn't quite thought out.  You didn't realize that the straps that would sit naturally and securely on the hips of a human wouldn't quite do so on a bare skeleton, so the set-up for this required a bit of improvisation on your part, but Sans didn't know the difference.  As far as he was concerned, it was your plan all along to go over to your desk, grab a roll of duct tape, come back, and start using long strips of it to secure the vibe to the most sensitive part of his pelvis.  You tested the tape a few times, added a few more strips until you were satisfied that it was secure, and then patted your boyfriend's knees.  "Alright!  All done there.  How's it feeling, sugar skull?"

 

You didn't hear a response.  Did he--?  Yes.  Yes, he did.  You heard snoring.

 

God damn it, Sans.  God damn it.  He deserved what he was about to get.  The cruel mistress you were, you reached for the controls and turned the vibrator up to high all at once.  You weren't sure how strong that was sensation-wise, having never needed to go past medium speed yourself, but the sound it made was atrocious.  Sans shot bolt-upright and instinctively tried to shove his hands over where the vibrator was rattling against his bones with a sound fairly comparable to a motorcycle revving its engine, but they just tugged uselessly against the scarf you'd tied them with.  He looked horrified.  You, on the other hand, were laughing so hard you could hardly get your hands to the controls to turn it back down to low.

 

" _Sans_!" you wheezed.

 

He replied with an equally scandalized exclamation of your name, followed by, "why would you do that?"  
  
"I-- heheheh!-- you were falling asleep!"  You wiped joyful tears from your eyes.  The much lower purr of the low-speed motor against his bones was significantly less funny, at least, so you were managing to calm down.  "Oh, man.  That was great.  Now I'll ask you again.  How's it feeling?"

 

Sans sighed softly, slowly letting himself relax (but not all the way-- never again, with a vibrator taped to his crotch).  "like i was just woken up in a very jarring way.  but, uh, as far as the...  butterfly dick thingy goes, pretty nice right now."  
  
"Mmm, good."  You straddled him and kissed him.  The low purr of the vibrator couldn't do much for you through the fabric of your shorts, but it was nice nonetheless to share the sensation with him at least in part, especially while you were kissing him. He seemed to enjoy that aspect of it, too. Once you were on top of him, he wriggled his hips and pulled himself up a little by his wrists so that his pelvis was aligned with yours, and he was more receptive to your kisses than you would have thought he'd be after you buzzed him awake. Of course, Sans, of all people, could always appreciate a bit of mischief.  Now that he seemed to be a little more in the mood, you broke away from your kiss and turned the vibration up a notch.  This time, it made a pleasant hum, like an instrument of your own making, and Sans was on vocals.  He moaned and pulled futilely against his restraints.

 

"ohhh, _fuck_. i didn't think a little buzzing sensation could feel so... so... so _much_." he chuckled, a blush spreading across his face as his hips pressed desperately up against you.  "shit, i don't know what i'm saying, i can hardly think right now."

 

"Right? Doesn't that feel amazing?" He nodded quickly in confirmation, so you let it continue at that speed and busied yourself with touching him.  Now that you didn't have to directly stimulate him, you could focus a lot more on what you were doing with your hands.  You made a broad stroke up his sternum with your hand held flat, then curved your fingers slightly on the way back down so that they caught a little bit in the spaces between his ribs. You could tell how aroused he was in the warmth of his bones, a direct result of his excited magic. Between your touch and the sex toy you were using on him, Sans could already feel it beginning its steady climb towards its first climax.  Since he already looked too wrecked to talk on his own, you continued prompting him.  "I'm going to turn it up a little more, okay?"

 

His eyes widened a little at that.  "a little more? jesus, babe, do you like it when i'm premature?"

 

"Maybe a little," you admitted mischievously.  "But hey, 'premature' is relative.  Don't hold back.  The first one is the quickest and easiest."

 

"the first...?"

 

"Orgasm."

 

"oh. well. i don't know how many i can have..."

 

You quietly shushed him while you reached for the controls again.  "Well, that's what we're here to find out, isn't it?"

 

"i, heh, i guess so."  His face scrunched up and he strained against your scarf as if he were trying to break free when you turned up the intensity.  It was approaching the strength you'd startled him with earlier, but now he seemed to be enjoying every moment of it.  You held him by the cheek, turned his head so you were face-to-face, and kissed him hungrily.  He could hardly kiss you back, he was so busy panting and moaning.  You could feel the huffs of breath escaping his nasal cavity, feel a faint buzzing resonating all the way from his pelvis to his teeth, or maybe that was just his excited magic, you could hardly tell.  He caught your lower lip between his teeth for a moment, which was the closest he could get to kissing you like a human would, and then he broke away with a whimper.  You'd been with him long enough to know what that meant.  The only time his voice was ever high-pitched was when he was about to cum.  Just to help him along, you wedged your knee between his legs and used it to push the vibrator more firmly into his pelvis.

 

That was enough to send him over the edge.  He let out a hoarse cry and, without the option of wrapping his arms around you, wrapped his legs around you, instead.  You could feel everything, from the waves of heat rolling off his bones to his gently bucking hips to the strong vibrations of the toy you were using on him.  You could tell when the sensitivity hit him, because he started laughing breathlessly, unable to do anything but clench his thighs around you and rock from side to side while the vibe, heedless to the end of his climax, continued just as strongly as before.  He looked so helpless, so happy and so sweet all at once, your heart ached for him more than your sex did.  You stifled his laughter with a kiss, felt him, tense and shuddering, in your arms, before he fell slack and panted for air.

 

"oh...  oh, stars, babe."  He groaned, shifting his hips away from your knee.  You weren't going to relent now, but you did show him a little mercy by turning down the vibration intensity.  "thanks, i just...  holy shit, heheh.  it got really, really sensitive for a minute.  now i hardly feel anything down there."

 

"Trust me.  You will," you quipped.  Proud of your work so far, you held him by the cheek and kissed him, softer than before.  He could just muster the energy to kiss you back.  You kissed him gently until the harsh puffs of breath against your lips became gradually more slow and even.  And then you got back to work on him.  
  


This time, after a few comforting strokes over his sternum, you left his ribs alone and focused on his pelvis instead.  You always found that part of him fascinating.  You were entranced by it.  You would, without shame, watch him get dressed in the morning and take in the dramatic curves and valleys of his hips, the tapering of his spine into his tail bone, the holes and the hollow spaces and the point in front where he usually formed his cock, but laid bare before you as it was now, it was nothing but a sensitive crevice.  You didn't know the anatomical terms for all those things, but god knew you would one day.  You didn't spare a single detail of him from your touch, you ran your hands over everything you were curious about, everything that you wished you knew the name for.  Your excitement built, and from the rising heat emanating off his body, you could tell that his did, too.  
  
  
"god, have i ever told you how...  how much i love it when you touch me?" he breathed with a strained smile to match his sweaty forehead.  "even...  mmm, normal, non-bedroom touching.  can't stand it from most people, but that first time you put your hand on my shoulder, i felt... i felt electric."  
  


"Electric, huh?"  You gazed at him bemusedly.  The vibe hummed a little louder against his bones when you turned it up.  He seemed recuperated now, ready to work up to another climax.  He gasped softly and turned his hips to one side as if to flinch away from the sensation, but all at once, the pleasure seemed to come back to him.  He shut his eyes and started lazily rutting against your crumpled bed sheets, while you pulled him back against you by his hips and started circling the vibe with your fingers. Your other arm wrapped around him to hold him by his rib cage.  He was so close to you, you could feel his every movement, from the tremors running through him to the rapid expanding and contracting of his rib cage.  When his second orgasm hit him, you held him so tightly that he hardly had the privilege of riding it out.  You stilled the thrusting of his hips, tangled his legs in yours so that he couldn't even kick or wrap them around something.  It was a strange and awesome sensation to be there, surrounded by you, totally immobilized with nothing to do but let the ecstasy crash over him.  It seemed to hit him twice as hard as his first climax, so hard that he seemed to forget all but three words: "oh," "fuck," and your name.  Consumed with excitement and desire, you rolled him over so that he was face-down and you were straddling his back, and he was just able to buck his hips throughout the last few moments.  His moans trailed off into whimpers, and his bones rattled with a weak shiver, both sure signs that he'd worked himself through to completion for the second time.

 

"Keep going, sweetheart," you whispered to him.  He looked like he needed the encouragement.  "Just keep going.  You're doing so good."

 

"i-i can't.  i can't, i can't," he wheezed.  "oh god, it's too much, it's too fucking much..."  He fought to lift his hips off the bed a little, and lifted you with them.  You were quick to respond.  


"Do you want to stop?"

 

"...  nooooo..."  The way he whined like a little kid having his toy-- pun intended-- taken away made you crack a smile.

 

"One more?"

 

"okay.  yeah.  shit.  one more."  He stayed in the position he'd ended up in, face down on the bed with his ass sticking up in the air.  You were more than happy to oblige, in fact, you were quite a fan of this new position.  With your hands holding on to his hips, you leaned down and planted slow kisses all the way up from his shoulder blades to the back of his skull.  He chuckled lowly under his breath.

 

"damn it.  can't pleasure you back from here.  can't even kiss ya' back.  what am i here for again?"  
  
"To be cute and have fun~" you reminded him sweetly.  "And you're doing a great job so far, so keep at it."

 

You knew that this last stretch would take the most time, but actually, you preferred that.  While Sans was still in that blissful lull between the oversensitivity from the previous orgasm and the coil of arousal enabling the next, you had plenty of time to kiss him in places you didn't usually get to, places other than his mouth or his cheeks.  You worked your way along his body with both your hands and your lips, the former leading the latter, caressing his ribs as you kissed his shoulders, then his spine as you kissed each of his ribs, then his hips as you kissed down his spine, then his femurs as you reached his tail bone.  You lingered there, that spot where, you'd learned the very first time you had sex with him, he was very sensitive in both the best and the worst way.  Touch him too roughly there, and it would hurt him, but touch him gently as you always did...  it was the kind of thing that could send him over the edge, or work him up to it when all else failed.  So you decided to try something a little new with him.  You kissed him there a couple more times, to prepare him, then you gently took it between your lips and flicked your tongue over it.  You didn't know if you were making a fool of yourself here, and probably wouldn't care that much if you did.  It wouldn't be the first time, and you could just laugh it off like always.

 

You absolutely were not making a fool of yourself.

 

"oh, baby, holy _shit_."  His knees buckled a little, but you quickly compensated.  You held his hips up where you could reach them and took more of him into your mouth.  It was unlike anything you'd ever done before, but it was the same basic concept.  Just don't use your teeth.  You found yourself doing so much of the work holding him up that your arms were getting tired, but it was so worth it, just to hear your boyfriend curse under his breath and whisper your name and ask you how the fuck you always knew how to make him feel amazing and tell you what a genius you were, how beautiful you were, how he wouldn't trade this or you for anything else in the world, wouldn't rather be anywhere but here, with you making him feel things he never felt before.  It would've made a passionate and well-composed speech, if it didn't come out so rambling, strung-together and interspersed with expletives whenever you used your tongue on him.  You were the one who decided when he'd had enough, and you decided it when his bones were warm enough to exceed the natural temperature of your mouth.  With a smirk, you pulled away and pried his legs apart a little more than they already were, in order to give yourself better access to the vibrator.

 

"You're so sweet, Sansy.  Are you just trying to get me to have mercy on you and let you cum?" you quipped.

 

"no.  i meant every word, but...  letting me cum _would_ be super nice of you," he huffed.  This time, he didn't even seem fully recovered from the last peak, and yet you could tell he was racing towards his next, and probably his last.  He looked and sounded absolutely wrecked right now.

 

"Well, you're lucky that I'm a super nice gal."  You were almost as excited for this as he was.  You fumbled for the controls one last time to turn the vibrator up almost as far as it would go.  The humming it made against his bones was loud, but not as loud as his startled and pleasured cry, his voice hoarse from overuse. You bent over him, reached around the side of his body and hooked your fingers around the vibe, simultaneously pressing it hard against his bones and moving it in the same quick, small circles you would use on your own clit.  It seemed to work just as well on him.

 

He could hardly form words this time on the lead-up to his climax, only broken syllables and desperate breaths, all the way up until it hit him, causing his body to crumple, his face to bury into your pillow and his hips to thrust downwards so hard that you were startled by the short and sudden fall.

 

"fu-hu-huck!"  With his voice muffled in the pillow, you couldn't tell whether he was laughing, or crying, or maybe both.  All you could tell was-- from the absurd blue blush you could just make out on his cheeks and shoulders and the fact that he'd never sounded quite so overwhelmed before, even during a really good orgasm-- it seemed that it was both his best and his last climax of the night.  He didn't even have to ask you.  Once his hoarse moans and the rolling of his hips petered out, you quietly turned off the vibe for the night and treated him to a whole second round of kisses, these ones slower and more gentle with no further intention than to calm and soothe him.  He was so still, and gradually, so relaxed, that you were sure he was going to fall asleep again in your arms.  It was a cute thought, but it didn't seem to be the case.  He didn't snore, just slackened and breathed into your pillow while you kissed him, almost like a stretching routine after hard exercise.

 

"Hey, baby."  You smiled, proud of yourself, but even prouder of him.  "How are you feeling?"

 

"...  this is, hands down, the best night of my life so far," came his muffled response.  You chuckled at that and massaged his shoulders for a moment.

 

"That's what I like to hear.  Alright, let's get you all freed up here."  You peeled away the tape and removed the vibrator first-- and marveled at how fascinating and convenient it was that he could orgasm like that without any resulting mess-- then worked on the scarf still holding his wrists to the bed post.  The moment he was untied, he used what had to be the last of his energy to flip over on to his back, sit up, wrap his arms around you, and kiss you like his life depended on it.  You were a little taken aback, but you smiled against his teeth and kissed him back just as passionately.  He leaned his weight into you until you were on your back and he was on top of you, then quietly pulled away, scooted downwards a little, and curled up there with his head on your chest.  You held him there gratefully for a moment, then chuckled softly.

 

"Sans?  We're lying down the wrong way," you pointed out gently.  "I can't reach the blanket."

 

"don't need it.  i got you."  Whether he was saying that you would warm him up, or he would keep you warm, it wasn't clear, but the fact still remained that you had a heavy naked skeleton on top of you that you were used to either having a T-shirt and shorts on while cuddling, or at least having a blanket pulled up to his shoulders.  Honestly?  Him being naked was making it hard for you to let your mind drift off to sleep.  Somehow, you had the idea to wrap your scarf around his neck.  That... wasn't much better.  But it made him smile, and that made it totally worth it.  "man.  what'd i ever do to deserve you?" he pondered softly.  Before you could even begin to list off all the reasons you loved him, he continued with a quick change of topic, "i'm gonna take that vibrator and pay you back for all that.  mark my words."

 

You blushed and held him a little tighter.  "Pft, okay, sure."

 

"'mserious.  gonna mark it on my calendar.  'make your girlfriend cum three times in a row.'"  That got you to laugh, and you ran a hand over the back of his skull.

 

"Sounds like a typical Friday night to me.  But thanks.  I would love that."

 

*          *          *

 

It was a few days later, and honestly, that passionate night with Sans had been pushed to the back of your mind by other things of much higher priority.  For example, your anniversary was coming up, and you wanted to make sure that Sans was available all day, because you had a _lot_ of stuff planned.  So, naturally, you took a look at his calendar app on his phone.

 

Maybe this meant you were a little nosey, but you always gained a certain sense of enjoyment from looking through Sans' stuff, getting little glimpses into the parts of his life you didn't always see, not because he hid anything from you, but because he probably considered it too mundane to interest you.

 

_05/04_

_star wars shindig w paps_

_05/17_

_anime night w al and undyne_

 

_05/26_

_make the bae squeal ~~2~~ ~~3~~ 5!!! times_

...  Well.  You guessed you weren't really all that surprised.  After all, Sans was known for keeping all his promises.

**Author's Note:**

> Always feel free to send us an ask over at marshmarrowsans.tumblr.com if you want to chat or have any fic or headcanon/imagine requests, we aren't formally a fic or imagines blog but we love to fill them out in our free time!


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